People fast for all sorts of reasons—from religious traditions and weight loss goals to hopes of boosting mental clarity and focus.
While going without food for a long stretch might sound extreme to some, others swear by fasting and include it as a regular part of their lifestyle.
But it’s definitely not for everyone. Experts recommend checking with a healthcare provider before starting any fasting routine.
If you’re curious though, a popular YouTube channel recently broke down what happens in your body during a 36-hour fast—and the changes are pretty wild.
What happens to your body during a 36 hour fast?
Four hours
A short 55-second clip uploaded by Wellness Wise on May 6 explains that around four hours after your last meal, your body stops processing food and digestion comes to a halt.
This stage kicks off what’s called the ‘catabolic phase,’ according to wellness expert Doctor Kiltz.
On his site, he explains that this phase is when your body starts breaking down stored energy into smaller units that your cells can use as fuel.
Eight hours
So you’ve gone eight hours without eating—what happens next?
Well, your blood sugar begins to dip, and your body switches from relying on incoming food to pulling energy from stored glycogen, according to the video.
12 hours
After hitting the 12-hour mark, glucose stores are nearly gone. That’s when your liver starts turning fat into fatty acids called ketones.
This change is known as ‘metabolic switching,’ and as the BBC explains, it’s one of the main reasons fasting can help with weight loss.
16 hours
At around the 16-hour point, your body begins a process called autophagy.
The Cleveland Clinic says this is when your cells break down old parts and reuse what’s still good to build new ones.
Dr. Hiltz notes that working out or lifting weights during this time can enhance autophagy, and if this process gets thrown off, it could potentially lead to things like abnormal cell growth.
24 hours
Going without food for an entire day is tough, but 24 hours into a fast is when serious cellular repair starts happening.
According to the YouTube breakdown—which has already racked up over 8 million views—your body is now running mainly on fat. This helps lower inflammation and makes your insulin response better.
30 hours
Once you pass the 30-hour mark, your growth hormone levels spike. This is believed to help preserve muscle mass and increase fat burning.
The video also mentions that this is the point when your body enters a ‘deep healing’ phase.
36 hours
At 36 hours, you’ve reached peak autophagy. Your body kicks into overdrive when it comes to cell repair, metabolism, and even tissue regeneration.
This reset can be extended for up to 72 hours if done safely and under the right conditions.
What do researchers say about fasting?
Experts are still debating whether fasting is a health miracle or overhyped trend. One big concern is that much of the evidence is based on animal studies, not humans.
“There [are] a lot of proposed benefits to [running on fats]. But a lot of the research hasn’t really [been borne out in] human beings. So we don’t see dramatic health benefits, certainly in the short term,” James Betts, a metabolic physiology professor at the University of Bath, told The Guardian.
Other research, such as reports from News In Health, suggests that people who fast for 16 hours or more regularly might be at greater risk for issues like gallstones.
“Many things happen during intermittent fasting that can protect organs against chronic diseases like type 2 diabetes, heart disease, age-related neurodegenerative disorders, even inflammatory bowel disease and many cancers,” says Mark Mattson, a neuroscientist at Johns Hopkins.
Bottom line? Everyone responds differently. So if you’re thinking about giving fasting a go, it’s best to talk to your doctor first and make sure it’s right for your body.